hello all,my 1999 gs300 has developed an intermitant misfire,my local toyota dealers, diagnostics revealed suspected oxygen sensors were faulty all four of them codes po135,po141,po155 and po161.they said it was unlikley that all four had gone ,there diagnostics couldnt get any more info and said i would be better off going to a lexus dealer.before i do does anyone have any sugestions please.